1. Field of the Invention
The present invention relates to a communication control apparatus, a communication control method, a node, and a communication system.
2. Description of the Related Art
Known methods by which a plurality of spatially distributed nodes can transmit data without collisions include time division multiple access (TDMA) and carrier sense multiple access (CSMA), the latter including carrier sense multiple access with collision avoidance (CSMA/CA) and carrier sense multiple access with collision detection (CSMA/CD). A discussion of these methods can be found in, for example, Waiyaresu LAN Akitekucha (Wireless LAN Architecture), edited by Matsushita and Nakagawa, Kyoritsu Shuppan, 1996, pp. 47, 53-59, and 69 (in Japanese).
A weakness of the TDMA system is that if the central administrative node that assigns time slots malfunctions, the entire communication system may be brought down. Various collision avoidance methods in which the individual nodes adjust their time-slot assignments autonomously, without the need for a central administrative server, have therefore been proposed, as in Japanese Patent Applications No. 2003-328530, No. 2004-257562, and No. 2004-257567. In these proposed systems, each node interacts with its neighboring nodes by transmitting impulse signals at regular intervals and adjusting the timing of these intervals according to the timing of the impulse signals received from the neighboring nodes, using a nonlinear model that forces the transmission timing away from the receiving timing. A group of interacting nodes can thereby establish non-overlapping time slots in which to transmit data without collisions.
Also disclosed in the above Patent Applications are similar systems in which the nodes interact continuously instead of interacting at discrete intervals, by continuous transmission of phase signals between nodes, and thereby autonomously acquire discrete time slots.
Systems in which the nodes interact continuously by transmitting phase signals can communicate more efficiently than systems in which the nodes interact by transmitting impulse signals at discrete intervals, because nodes in the former type of system can adjust their communication timing continuously, whereas a node in the latter type of system adjusts its communication timing only when it receives an impulse signal from another node. More specifically, in a system in which interaction takes place by impulse signals, the time slots obtained by mutually adjacent nodes tend to differ in length, which impedes efficient communication when, for example, a node relays data between two other nodes. As a result, the efficiency of utilization of the network is reduced.